1. Field of the Invention
This invention relates to an apparatus for preventing the one-sided lowering of a jacking mechanism including a plurality of jacks which are designed for simultaneous operation.
2. Description of the Prior Art
There is known a jacking mechanism including a plurality of jacks which are designed for simultaneous operation. It is widely used as, for example, a cargo handling device, or a device for lifting a motor vehicle when repairing it. A typical device of this type comprises a pair of jacks which are connected to each other by a device for raising and lowering the two jacks simultaneously. It also includes a horizontal table or tables provided on the jacks for carrying a load thereon. Each jack is provided with a substantially vertically disposed rack and a pawl which is engageable with the rack for holding the table in its raised position. The pawls are disengaged from the racks when it is necessary to lower the table or tables.
If any attepmt is made to lower the table or tables before the pawl of one of the jacks is disengaged from the rack, however, the jack stays in its raised position, while the other jack is lowered. This results in the inclination of the table or the positioning of the two tables at different levels of height and thereby in the inclination, displacement or dropping of the load on or off the table or tables. In any such event, the load is likely to cause a hazard to the jack operator.
Various kinds of safety devices are, therefore, known. They include a device which detects the position of the tables or tables and transmits a signal for actuating the device for lowering the table or tables only when the table is not inclined, or when the tables stay at the same level of height. This device is, however, complicated and sophisticated. It is expensive and adds considerably to the cost of the jacking device as a whole.
There is also known a device including a limit switch which functions to actuate the device for lowering the table or tables only when the pawls are disengaged from the racks. The device of this type which is now available, however, lacks reliability, as the terminals of the limit switch often fail to make proper contact, or as a limit switch actuator often fails to work properly. Moreover, it is necessary to use an explosion-proof device to prevent any fire from starting in a place where combusttible gas exist, or vapor of combustible liquid is likely to occur. It is expensive and adds undesirably to the cost of the jacking device as a whole.